Sen. Joe Manchin raced to a secure location in West Virginia just before Labor Day and fielded a high-level classified phone call about Syria from President Barack Obama’s top aide.

The aide, White House chief of staff Denis McDonough, laid out the administration’s case against the Assad regime, prompting Manchin to cancel a weeklong series of events back home and return to Washington to attend classified briefings.

The briefings helped solidify a decision in the Democrat’s mind: The White House’s plan to launch a military strike in Syria was precisely the wrong way to go.

“It never did rise to the level that the threat was imminent to the United States of America,” said Manchin, who drafted an alternative Syria plan with Sen. Heidi Heitkamp (D-N.D.). “And the reaction from our actions would have been a much greater risk to our country than an inaction at this time.”

Over the past two weeks, senior White House officials mounted the most intense bipartisan lobbying campaign they have ever conducted on Capitol Hill — with Obama administration officials speaking with more than 370 House members and at least 93 senators — to persuade lawmakers to back the president on Syria.

But according to many House and Senate members from both parties, the briefings were a flop: They raised more questions than answers and failed to persuade skeptical lawmakers to back a use-of-force resolution.

Congress is off the hook — for now — as Obama is pursuing diplomatic options with Russia before seeking any vote on military force in Syria. Secretary of State John Kerry traveled to Geneva on Thursday to meet with his Russian counterparts. If diplomacy fails, the Senate may resume consideration of a resolution, calling for “limited and tailored” strikes of up to 90 days while prohibiting the use of combat forces on the ground.

But the lingering questions from lawmakers span the gamut. Many express frustration that there is no clear answer on what would happen if the U.S. were to launch a “limited” military campaign against Syrian President Bashar Assad. The failure to win over lawmakers in both chambers underscores the bind the White House finds itself in: If diplomacy fails, Congress is unlikely to go along either.

Even supporters of military action in Syria are frustrated that the administration’s sales job floundered.

Tennessee Sen. Bob Corker, one of the administration’s closest GOP allies on Syria, said the briefings — and shifting messages from the White House — have been “so bad” that it makes it “really, really hard” to continue backing military force in the region. He said the administration has failed to make the case that attacking the Assad regime is in the national interest, adding that the lack of credibility the United States now has globally is “stunning.”

“Their message is just so muddled,” Corker told POLITICO Wednesday. “Different audiences, they stress different things. … They keep trying to find some footing that makes them feel good, or the audience feel good; it’s been the most muddled thing I’ve ever seen in my life.”

Arizona Sen. John McCain, a leading GOP proponent of military action in Syria, similarly criticized the Obama briefers, including Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, with whom he has frequently sparred.

“One reason is because they are not specific: They are not answering many of the questions,” McCain said when asked about the failure of the briefings to sway lawmakers. “Certainly, that was the case in the Armed Services Committee. And Gen. Dempsey doesn’t have a lot of credibility.”

The briefings for lawmakers often occurred in secure rooms in the U.S. Capitol Visitor Center with a range of high-level administration officials, including Kerry, Defense Secretary Chuck Hagel, National Security Adviser Susan Rice and Tony Blinken, the deputy national security adviser. But critics complain there are too many attendees asking a wide array of hard-to-answer questions and that few concerns are alleviated by the Obama officials’ presentations.

Administration defenders argue that the problem isn’t the briefings, which they say have been important venues offered by the White House to provide lawmakers with classified intelligence they ordinarily wouldn’t have access to. They say the growing opposition in Congress is rooted in the simple fact that lawmakers are hearing an array of concerns from constituents in their districts and states.

“Members are listening to the folks back home; folks back home don’t agree with the president,” said Senate Majority Whip Dick Durbin (D-Ill.), who voted for the use-of-force resolution in the Senate Foreign Relations Committee. “A majority of people oppose his position, and they’re hearing and listening to what is being said.”

California Rep. Xavier Becerra, chairman of the House Democratic Caucus, said the “evolving circumstances” in Syria have made it hard for the administration officials to clearly lay out scenarios for a Syria attack. He said lawmakers raise questions about the operational details of a strike, including the assets and artillery that would be targeted and the number of casualties the United States could expect.

“It’s those kind of questions where it’s tough to give precision,” Becerra said Wednesday.

Sen. Dianne Feinstein (D-Calif.), the Intelligence Committee chairwoman, was visibly frustrated by what she views as a clear attempt by congressional Republicans to “discredit” Obama — something she argued Democrats did not do in the run-up to the 2002 authorization of the Iraq War.

“I think it’s unfair to be candid with you,” Feinstein said.

Senior administration officials argue that they have held an unusually open dialogue with Congress to lay out the most compelling case that Assad used chemical weapons and that Washington had to act.

“The president acknowledged from the beginning that this would be a challenge,” White House spokesman Jay Carney said Wednesday. “We have presented a great deal of information to members of Congress, both in the House and the Senate, and we are continuing to do that today in briefings and will continue to do that moving forward. Congress has only been back in town for a day and a half.”

But at this point, the White House does not have majority support for the use-of-force resolution in the Democratic-led Senate, according to several Democratic sources, and the GOP-led House is an even tougher climb. To get to 60 votes to break a filibuster and 50 votes for passage in the Senate, Obama and his team will have to persuade enough Republicans to go along with the resolution.

That puts a senator like Susan Collins in the administration’s crosshairs.

But after spending three hours at Vice President Joe Biden’s home Sunday night for dinner and sitting through several other classified briefings, the moderate Maine Republican still has more questions about how the U.S. would avoid getting dragged into a protracted civil war in Syria.

“My concern that that may happen is heightened when the president and his advisers, in response to questions that I and others have raised about what would happen if Assad used chemical weapons again, even if there were a small attack, the answer from the administration has been that they would launch another strike.

“So where does it end?” Collins said.

Lawmakers who attended several of Biden’s briefings said privately that the vice president spoke almost uninterrupted for more than 90 minutes at the sessions, starting with a recounting of the evidence that Assad used chemical weapons and moving through the administration’s case for military action. He asked one set of House Republicans to keep an “open mind” on force, though he did not appear to win many votes in his briefings.

“It’s hard to say they did a good job because they don’t have a strategy that they’ve articulated not just to Congress, but to the American people,” said Sen. John Hoeven (R-N.D.).

Sen. Angus King (I-Maine), who has yet to take a position on the use-of-force resolution, said the briefings have yet to detail the reprisals that could occur if the United States were to engage in Syria.

“Nobody can give you any guarantees,” King said. “That’s why a briefing can’t be definitive because these are questions that can’t have definitive answers.”

But Manchin was able to come to a definitive answer on his own position. Manchin said the briefings “crystallized” that the administration lacked a “resolve” to take out Assad, there was no clear way to secure his chemical weapon stockpile, no guarantee that the airstrikes wouldn’t accidentally cause more collateral damage and no assurances that the Muslim world wouldn’t rise up and spark a larger regional war.